1. Field of the Invention
The invention relates to a hydrophobic precipitated silica which is prepared without heat treatment, has a high level of whiteness and can be worked well in silicone rubber formulations and causes a high reinforcing action in the silicone vulcanizates.
2. Discussion of the Background
The treatment of finely divided solids, metal oxides, and silicates with organosilicon compounds, such as organopolysiloxanes, is described, for example, in DE 30 85 905. The heat treatment process is carried out under an inert atmosphere of nitrogen. Hydrophobic silicates are also described in, for example, DE 24 35 860, DE 22 42 728, and DE 25 13 608.
In the documents above, hydrophilic silicates and precipitated silicas are hydrophobicized by reacting them with organosilicon compounds. Examples of hydrophobicizers include organohalosilanes and organopolysiloxane compounds.
DE 26 28 975 and DE 27 29 244 describe preparing hydrophobic silicas by reacting a hydrophilic precipitated silica having low water absorbency with silicone oil or dimethyldichlorosilane, respectively. In the process according to DE 26 28 975, the reaction is carried out with the hydrophobicizer (silicone oil) being added to the dry precipitated silica; in the process according to DE 27 29 244, the hydrophobicizer (dimethyldichlorosilane) is introduced directly into the precipitated silica suspension. In both cases, the hydrophobicizing step is followed by heat treatment at elevated temperatures, specifically between 200 and 400° C.
A disadvantage of the above process is that the precipitated silica thus hydrophobicized becomes discolored at the required process temperatures. The discoloration of the hydrophobicized silica is particularly inconvenient when it is added to silicone formulations, i.e., when the hydrophobic precipitated silicas are added to silicone rubber formulations or to defoamer mixtures based on silicone oil.
A further disadvantage of the heat treatment at temperatures of 200-400° C. is its relatively high cost, which makes the hydrophobicized silica comparatively expensive.
It is possible to use reflectance as a measure of the discoloration. In a reflectance measurement, the diffuse reflection power of a sample is investigated. The higher the diffuse reflection power of a sample, the higher its reflectance and thus the higher the whiteness of the sample.
Precipitated silicas generally have a reflectance of not more than 97%. It would be desirable to hydrophobicize silicas in such a way that the reflectance of the original silica is retained.